FIG. 18 shows a first conventional example (JP2002-252970A). An input 1 is switched by a switching element 7, and a voltage of square wave resulting from the switching is smoothed by a reactor L(26) and a capacitor Cf(10) and outputted. In the process of output, a voltage obtained by dividing an output 11 is compared with a sawtooth wave outputted from a sawtooth wave generator by a comparator and a switching signal according to this comparison result is applied to the switching element 7. When the comparison result is applied, a counter is operated in synchronization with a vertex of the sawtooth wave, a switch SW1 is opened/closed, a time constant of a time constant circuit made up of R0 and C is adjusted according to the presence/absence of a resistor R1, signals of frequency f1, f2 generated from the sawtooth wave generator are switched sequentially and given to the comparator. The noise level is reduced by distributing peaks of switching noise in the frequency domain, sequentially switching between the switching frequencies and thereby distributing noise energy.
Furthermore, FIG. 19 shows a second conventional example (JP2006-288104A). The rising edge of a drive pulse that drives a power switching element is repeatedly shifted by a basic pattern made up of three different shift amounts φ0 to φ2 for a period T1. In this way, the switching frequencies created by the rising edges of these drive pulses are distributed. Furthermore, a distribution frequency which is the reciprocal of the repetition period T of this basic pattern is set to an audible frequency or higher.